Generally, there are two kinds of developers for electrostatic latent image development; a one-component type developer and a two-component type developer. Among them, the two-component type developer method is more frequently used due to a point that provision of charge to toner is relatively stable compared to the one-component developer because the so-called carrier which provides charge to toner is mixed with toner and due to a point that, while a color copying machine is spreading remarkably, a magnetic material is not necessary for toner and that the color of the magnetic material does not deteriorate the tone of outputted image.
The two-component type developer is composed of toner and carrier. The carrier is generally classified into electroconductive carrier and insulating carrier. In many cases, however, from the viewpoint of durability and ability to provide an electric charge, resin-coated carriers belonging to the insulating carrier are used. A technology to laminate the surface of this carrier with resin is disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter, referred to as Japanese Patent O.P.I. Publication) Nos. 13954/1972 and 208765/1985.
The two-component developer needs to be stirred in a developing apparatus so that its carrier and toner are mixed and toner is thereby charged for developing.
As an electroconductive carrier, an iron powder carrier and an iron oxide powder carrier are frequently used. In the case of the iron powder carrier, the amount of electric charge provided to the toner tends to be unstable so that there is a problem that fogging occurs on a visible image formed by the developer. The causes for this are that development bias electric current is reduced due to the increase of electric resistance of carrier caused by the adhesion and accumulation of toner particles on the surface of the carrier in the course of stirring and mixing in the developing apparatus and that the amount of electric charge provided to the toner becomes unstable because the surface of the carrier is covered with toner. Accordingly, since a developer composed of the iron powder carrier deteriorates in a small number of using cycles, it is necessary to replace with a new developer earlier.
Therefore, in many cases, resin-coated carrier, wherein the surface of magnetic particles is coated with resin, is used.
This carrier can control the amount of electric charge provided to the toner by selecting resin for coating. In addition, fusion of toner onto the surface of the carrier hardly occurs. Therefore, the advantages are that the amount of electric charge provided to the toner becomes stable and that the developer is excellent in terms of durability compared with the iron powder carrier.
To the contrary, however, different problems which do not occur in the iron powder type carrier occur so that conventional resin-coated carriers cannot produce the desired performance. A major problem of the resin-coated carrier is peeling of the resin-coated layer which occurs when carrier is stressed in the developing apparatus. When the resin-coated layer is peeled off, the ability to provide electric charge to the toner becomes unstable resulting in fogging on visible images formed by the developer. In addition, concurrently with this, the core material of the carrier is exposed so that the electric resistance of the carrier is reduced. The reduction of the electric resistance of the carrier causes thin blurred lines and characters due to excessive development and adhesion of the carrier onto the photoreceptor.
When the surface of the carrier is coated with resin, it is easily influenced by the conditions of resin-coating device and resin-coating circumstances, especially humidity. Accordingly, even with strict control, it is difficult to coat resin on the surface of carrier uniformly and to make the performance of developer stable over a long period. It is the current status that satisfactory performance has not been obtained.
In addition, in order to obtain higher image quality, the particle size of toner is reduced. In the case of the two-component type developer, it may also be necessary to reduce the particle size of carrier in accordance with the particle size of toner in order to provide electric charge sites on the surface of the carrier. However, as reduction of the particle size of carrier is advanced, it becomes more difficult to form a uniform resin-coated layer. Therefore, the mechanical strength of the resin-coated layer becomes unstable so that the above-mentioned shortcoming becomes more obvious. As a result, a problem for practical use problems become greater.
The above-mentioned problem occurs in both the contact development method and the non-contact development method. In the case of the contact development method wherein a magnet brush composed of toner and carrier is brought into contact with a photoreceptor for developing, the above-mentioned problem of resin-coated carrier occurs prominently in a developing apparatus for high speed development. In order to conduct high speed developing, it is necessary to mix and stir supplied toner and carrier at high speed in a developing apparatus. Therefore, carrier receives extremely large stress at a mixing and stirring section. Concurrently with this, in order to conduct high speed developing, it is necessary to rotate developing sleeve at high speed. Therefore, carrier also receives extremely large stress at a development nip section between the development sleeve and the photoreceptor.
In order to reduce the above-mentioned excessive stress, mixing and stirring speed is slightly adjusted, the development nip distance is widened and the rotation speed of developing sleeve is restricted by enhancing toner density. However, these countermeasures cause the occurrence of toner scattering and fogging due to the incapability of providing sufficient electric charge to toner and low image density due to insufficient development material conveyed to the developing region.
In addition, in the case of a non-contact development method wherein development is conducted without contact of the developer layer to the photoreceptor, toner images once developed are not disturbed by contact of the magnetic brush, resulting in enhancement of the image quality. On the other hand, however, developability tends to be inferior compared to the contact developing method. As a countermeasure therefor, it is necessary to narrow the distance between the photoreceptor and the development sleeve. In order to introduce a stable amount of developer to this narrow developing region, it is necessary to set the developer layer uniform and reduce the thickness of it as much as possible. For this purpose, a thin layer forming method by means of a thin layer forming member such as stiff stick magnetic material as proposed in Japanese Patent O.P.I. Publication No. 50184/1990 is effective for forming stable layer thickness. However, though formation of a thin layer by means of a thin layer forming member such as a stick magnetic material has a merit to form a stable layer, stress given to the developer by a member forming the thin layer becomes excessive.
As a countermeasure therefor, as shown in Japanese Patent O.P.I. Publication No. 232362/1974, adding hydrophobic silica fine particles in a resin-coating layer for carrier is proposed. In this case, as the developer is used, the silica fine particles added moves from the original position to the surface of toner so that electrification of toner is hindered. Therefore, this countermeasure cannot be said a sufficient countermeasure.